1. Field of the Invention
The present invention relates to data storage systems.
2. Background Art
Greater quantities of data are being generated and stored by computer systems. Some data, such as intermediate calculations, is discarded almost immediately. However, an increasing amount of data must be stored for extended periods of time. This period of time may range from minutes to years depending on the type of data, type of application which generates or uses the data, and requirements imposed by government regulations, business practices, historical needs, and the like.
Long term data storage must often meet apparently conflicting performance criteria. Ideally, all storage should have a low cost-per-bit of data stored. This stored data must be stored and readily retrievable in a brief period of time. This time may be measured in a variety of ways, such as the time before the first data is transferred, the amount of data received each second, the total access time for a given quantity of data, and the like. Space required to store the data becomes an increasingly important parameter as the amount of data increases. Additionally, high reliability is required for stored data. This reliability can be achieved through the use of highly dependable storage devices, through the use of systems for correcting or recovering data, through redundancy of storage equipment, and the like.
Traditionally, no one type of storage device has been able to satisfy cost, speed, density and reliability requirements. High performance magnetic disks have been used as a first level of data storage due to their high data rates and low access times. However, high performance magnetic disk systems tended to be relatively expensive and low density. Secondary storage for backup, archiving and very large storage requirements has typically been provided by magnetic tape. Magnetic tape offered greater storage density and lower cost-per-bit than high performance magnetic disk systems, but often suffered from greatly increased data access time.
Increasingly, a wide gap is developing between the characteristics of high performance disk systems and tape systems. Various attempts at providing an intermediate level of long term storage have not met with great commercial success. One technique is to use a collection of inexpensive, low performance disks. An example of low performance disks are those meeting the ATA standard such as those used in many personal computers. However, implementing groups or arrays of these inexpensive, low performance disks exhibit certain difficulties. For example, inexpensive disks are typically incompatible with high speed media interfaces such as Fibre Channel (FC), SCSI, iSCSI, ESCON and the like. Another problem with arrays of inexpensive disks is that such arrays tend not to provide any increase in storage density over high performance disk systems. Yet another problem with arrays of inexpensive disks is that such arrays are typically designed for special purpose use only and are not compatible or configurable for use with a large number of high performance computer interfaces or applications.